I. Field of the Invention
The present invention relates to telecommunications, and more particularly, to wireless communications.
II. Description of the Related Art
Wireless communications systems provide wireless service to a number of wireless or mobile units situated within a geographic region. The geographic region supported by a wireless communications system is divided into spatially distinct areas commonly referred to as “cells.” Each cell, ideally, may be represented by a hexagon in a honeycomb pattern. In practice, however, each cell may have an irregular shape, depending on various factors including the topography of the terrain surrounding the cell. Moreover, each cell is further broken into two or more sectors. Each cell is commonly divided into three sectors, each having a range of 120 degrees, for example.
A conventional cellular system comprises a number of cell sites or base stations geographically distributed to support the transmission and reception of communication signals to and from the wireless or mobile units. Each cell site handles voice communications within a cell. Moreover, the overall coverage area for the cellular system may be defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to ensure, where possible, contiguous communication coverage within the outer boundaries of the system's coverage area.
Each base station comprises at least one radio and at least one antenna for communicating with the wireless units in that cell. Moreover, each base station also comprises transmission equipment for communicating with a Mobile Switching Center (“MSC”). A mobile switching center is responsible for, among other things, establishing and maintaining calls between the wireless units, between a wireless unit and a wireline unit through a public switched telephone network (“PSTN”), as well as between a wireless unit and a packet data network (“PDN”), such as the Internet. A base station controller (“BSC”) administers the radio resources for one or more base stations and relays this information to the MSC.
When active, a wireless unit receives signals from at least one base station or cell site over a forward link or downlink and transmits signals to at least one cell site or base station over a reverse link or uplink. There are many different schemes for defining wireless links or channels for a cellular communication system. These schemes include, for example, time-division multiple access (“TDMA”), frequency-division multiple access (“FDMA”), and code-division multiple access (“CDMA”) type-designs.
In a CDMA scheme, each wireless channel is distinguished by a distinct channelization code (e.g., spreading code, spread spectrum code or Walsh code) that is used to encode different information streams. These information streams may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver may recover a particular stream from a received signal using the appropriate Walsh code to decode the received signal.
Each base station using a spread spectrum scheme, such as CDMA, offers a determined number of Walsh codes, and consequently, a corresponding number of users, within each sector of a cell. In the CDMA 2000 1X, for example, the number of Walsh codes made available by each sector may be defined by the radio configuration (“RC”) employed by the base station. Consequently, the number of Walsh codes available for an RC3 assignment is 64, for example, while an RC4 assignment, in contrast, supports 128 Walsh codes. Under certain conditions, such as when the majority of users are in benign RF environment, the users are concentrated in the area near antenna or majority of the users are stationary, etc., the capacity of CDMA 2000 1X may exceed the Walsh code capability of an RC3 assignment. RC3 assignments may also be exceeded when technologies, such as transmit diversity, an intelligent antenna(s), and/or a selectable mode vocoder(s) are introduced.
The number of Walsh codes made available by a base station takes into consideration the transmit power requirements associated with the selected RC assignment. For example, an RC4 assignment requires a relatively longer spreading code and has a greater transmit power requirement than an RC3 assignment, which may be a relatively shorter spreading code. Thusly, while increasing the number of Walsh codes by selected a higher RC assignment on the downlink may increase voice capacity, the robustness of the modulation may be reduced. For the purposes of the present disclosure, reference to voice capacity also includes circuit switched services similar to voice, such as video, for example. An RC4 assignment may therefore degrade capacity, for example, by supporting a weaker coding rate than an RC3 assignment.
To maintain the efficacy of the base station's operation, an increase in the signal to noise ratio (“SNR”) may be needed if a higher RC assignment may be selected. To raise the SNR, however, an increase in transmit power may also be necessary. Consequently, a tradeoff exists between the power efficiency of the base station based on the RC configuration employed and the length/number of spreading codes made available within each sector of a cell.
As a result, a need exists for increasing voice capacity on the downlink without unduly influencing the power efficiency of the base station.